Problem: On the first 3 exams of her physics class, Tiffany got an average score of 77. What does she need on the next exam to have an overall average of 79?
Let her score on the next exam be $x$ The sum of all of her scores is then $3 \cdot 77 + x$ The same sum must also be equal to $4 \cdot 79$ Solve: $x = 4 \cdot 79 - 3 \cdot 77 = 85$.